In this paper a numerical investigation is carried out to determine the optimal location of a heat source mounted in a substrate in a cavity filled with a dielectric fluid of Pr = 25. The opposite vertical wall and the horizontal walls are assumed to be isothermal and adiabatic, respectively. The governing steady state partial differential equations for the fluid and solid regions are solved simultaneously using a control volume formulation. The heat source/fluid thermal conductivity fixed while the substrate/fluid thermal conductivity ratio is varied from 10 to 1000. The objective is to maximize the thermal conductance between the heat source and fluid that is equivalent to minimize the heat source temperature. The predications show that the average Nusselt number of the heat source increases with increasing Rayleigh number and the decrease in heat source spacing from the cavity bottom surface. It is shown that increasing the substrate/fluid thermal conductivity ratio decreases the average heat source Nusselt number and increases its thermal conductance. For 7 Ra b ≤10 , the optimum location is not critical with maximum thermal conductance at s/b=2.5. The optimal location migrates toward the lower corner of the cavity for 7 Ra b >10 at Rs